1. Field of the Invention
The present invention relates to a control method of an AC/DC power converter for input current harmonic suppression. More particularly, the present invention relates to the control method for applying to the AC/DC power converter without detecting an AC voltage so as to adjust an input AC current to approximate nearly as a sinusoidal wave and to obtain an unity power factor, and to supply an output of an adjustable DC voltage.
2. Description of the Related Art
Power converters have been widely used in many areas recently. These power converters include AC/DC, DC/DC and DC/AC converters. Conventionally, the AC/DC power converter is configured by a diode rectifier. There are advantages of simplified configuration and reduced cost in using the diode rectifier. However, a DC side of the diode rectifier cannot be controlled, and a great amount of input harmonic components and poor input power factors occur in its AC side.
In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC1000-3-2, and IEC1000-3-4 etc., have been established. In this way, the modern power electronic equipment need to meet the requirements for low input harmonic distortion and high input power factor. Recently, a variety of power factor correctors are developed to solve the harmonic problems caused by the AC/DC power converter.
Referring to FIG. 1, a schematic circuitry of a conventional power factor corrector is illustrated. Generally, the power factor corrector includes a diode rectifier 10, an inductor 11, a power electronic switch 12, a diode 13, a DC capacitor 14 and a controller 15. Control methods for the power factor corrector are well known and described in U.S. Pat. No. 6,650,554 and U.S. Pat. No. 6,388,429, for example. The output DC voltage of the power factor corrector can be controlled by controlling the power electronic switch 12. The output DC voltage of the power factor corrector is higher than a peak value of an input AC voltage. An input current approximated nearly as a sinusoidal wave and an unity input power factor can be obtained at an input AC side of the power factor corrector. The conventional control method for controlling the power factor corrector employs a detected output DC voltage for regulating the output DC voltage so as to determine a reference amplitude of the input AC current. Subsequently, a detected AC voltage is employed to determine a reference waveform of the input AC current. The reference waveform multiplies the reference amplitude, thereby obtaining a reference signal of the input AC current. Subsequently, the reference signal and a detected input AC current are operated in a closed-loop control to produce a modulation signal. Finally, the modulation signal is sent to a pulse-width-modulation circuit and a driving circuit to produce a driving signal for the power electronic switch 12. In this way, the conventional control method for the power factor corrector disadvantageously requires to detect three signals, including the output DC voltage, the input AC voltage and the input AC current.
Generally, an AC/DC power converter must employ a power converter having a bridge configuration. Referring now to FIGS. 2a and 2b, schematic circuitry of conventional single-phase AC/DC power converters applied to a single-phase AC power system in accordance with the prior art are illustrated.
Still referring to FIG. 2a, the conventional single-phase AC/DC power converter having a half-bridge configuration is disclosed. The half-bridge configuration of the single-phase AC/DC power converter includes a power electronic switch set 20, a pair of capacitors 21, 22, an inductor 32 and a controller 24. The power electronic switch set 20 has two power electronic switches. The capacitors 21, 22 have the same capacitance. The controller 24 can control the power electronic switch set 20, thereby controlling the AC/DC power converter to receive an input AC current supplied from an AC power source through the inductor 23. Advantageously, the input AC current is approximated nearly as a sinusoidal wave and in phase with the input voltage of the AC power source. Consequently, the harmonics in the AC/DC power converter can be suppressed, the power factor is nearly unity, and the output DC voltage can be controlled.
Referring again to FIG. 2b, the conventional single-phase AC/DC power converter having a full-bridge configuration is disclosed. The full-bridge configuration of the single-phase AC/DC power converter includes a power electronic switch set 30, a capacitor 31, an inductor 32 and a controller 33. The power electronic switch set 30 has four power electronic switches. The controller 33 can control to switch the power electronic switch set 30, thereby controlling the AC/DC power converter to receive an input AC current supplied from an AC power source through the inductor 23. Advantageously, the input AC current is approximated nearly as a sinusoidal wave and in phase with the input voltage of the AC power source. Consequently, the harmonics in the AC/DC power converter can be suppressed, the power factor is nearly unity, and the output DC voltage can be controlled.
Referring to FIG. 3, a schematic circuitry of a conventional three-phase AC/DC power converter applied to a three-phase AC power system in accordance with the prior art is illustrated. The three-phase AC/DC power converter includes a power electronic switch set 40, a capacitor 41, a three-phase inductor set 42 and a controller 43. The power electronic switch set 40 has six power electronic switches. The controller 43 can control to switch the power electronic switch set 40, thereby controlling the AC/DC power converter to produce a balanced three-phase sine-wave currents on the three-phase inductor set 42. Advantageously, phases of the three-phase sine-wave currents are identical with those of the input voltages of a three-phase power source. Consequently, the harmonics in the three-phase AC/DC power converter can be suppressed, and the power factor can be improved to nearly unity.
The conventional control method for both the single-phase AC/DC power converter and the three-phase AC/DC power converter employs a detected output DC voltage for regulating the output DC voltage so as to determine a reference amplitude of the input AC current. Subsequently, a detected AC voltage of the AC power source is employed to determine a reference waveform of the input AC current. The reference waveform multiplies the reference amplitude, thereby obtaining a reference signal of the input AC current. Subsequently, the reference signal and the detected input AC current are operated in closed-loop control to produce a modulation signal. Finally, the modulation signal is sent to a pulse-width-modulation/driving circuit to produce a set of driving signals for the power electronic switch sets 20, 30, 40. In this way, the conventional control method for the single-phase AC/DC power converter and the three-phase AC/DC power converter disadvantageously require to detect three signals, including the output DC voltage, the input AC voltage and the input AC current.
Even though the conventional control methods of the AC/DC power converters can suppress the harmonic components of the input AC current and improve the power factor, the controller must detect the output DC voltage and the input AC voltage to determine the reference signal. Subsequently, the input AC current is detected and operated in closed-loop control to obtain a sine-wave input AC current. Advantageously, the sine-wave input AC current is in phase with the input voltage of the AC power source. However, the conventional control methods for the AC/DC power converter disadvantageously require detecting three signals, including the output DC voltage, the input AC voltage and the input AC current. Accordingly, the control circuit can be complicated and cannot be normally operated due to fluctuations in frequency of the AC power system.
The present invention intends to provide a simplified control method of an AC/DC power converter for suppressing the input current harmonics. The control circuit only detects two signals from the output DC voltage and the input AC current. Additionally, the control method can be normally operated under fluctuations in frequency of the AC power system for controlling an input AC current to approximate nearly as a sinusoidal wave and a unity power factor, and to supply an adjustable output DC voltage.